In recent years, with miniaturization and technological advance of electronic devices, how to take heat dissipation measures has become an important problem. Thus, in order to achieve the high thermal conduction of a resin which constitutes a substrate on which an electronic component is implemented, a method of adding particles with a high degree of thermal conductivity as fillers to the resin has generally been used. Particles with a higher degree of thermal conductivity have been used as electrically insulative fillers added to form an insulative resin with high thermal conductivity.
For example, a method of using aluminum nitride having a high degree of thermal conductivity as a filler has been examined. However, aluminum nitride is extremely unstable against water and is easily reacted with water in atmospheric air to generate ammonia to generate aluminum hydroxide on the surface of an aluminum nitride powder.
Accordingly, it has been known that thermal conductivity is deteriorated with generation of aluminum hydroxide even when powdery aluminum nitride is stored in atmospheric air for a long term or dispersed in a resin or the like. Thus, for example, a method of improving water resistance by coating the surface of aluminum nitride with an organic matter or an inorganic matter has been known.
However, an organic matter or an inorganic matter on an aluminum nitride surface has sometimes inhibited thermal conduction to decrease the degree of the thermal conductivity of a composite material per se in the case of making composite aluminum nitride particles by coating the aluminum nitride particle surface with the organic matter or the inorganic matter (such as metal oxide, glass, or a metal salt). Further, a method of using α-transformed alumina having good thermal conductivity for a surface-coated inorganic matter in a coating layer to reduce the effect of inhibition of thermal conduction by surface coating has been known (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 4-175209).
However, a coating layer including crystalline α-alumina has insufficient water resistance since many cracks are generally generated during sintering. Thus, a method of reducing the occurrence of cracking by controlling the amount of an oxygen gas in a sintering gas when a crystalline α-alumina layer is formed on an aluminum nitride surface has been known (e.g., see JP-A 2005-225947).